We have found it most valuable, although our primary concern remains the comparative metabolism of larval and adult forms of schistosomes, to extend the range of test systems to include larval and adult forms of filariae as well as culture and bloodstream forms of the African trypanosome, Trypanosoma (Trypanozoon) brucei rhodesiense. As obligate parasites which for the most part occupy similar environmental niches within their mammalian hosts, the species we have chosen for study have been found to share metabolic peculiarities not operative in mammalian cells. It remains to be determined whether those stages that are parasitic in invertebrate intermediate hosts also possess aspects of metabolism that differ from the analogous ones in their hosts. The detection of such differences could lead to the development of chemotherapeutic agents with increased selective toxicity for the parasites. Such comparative biochemical studies have led to the discovery that the purine nucleoside antibiotic tubercidin, administered intraerythrocytically, can arrest schistosomiasis mansoni and japonica in monkeys with no obvious host toxicity. Furthermore, the nitrovinylfuran derivative, SQ 18,506, originally found to be schistosomicidal, is also trypanocidal, but with a different mechanism of action. These and other new findings are serving as the bases for continuing investigations.